Using Multiple Ignition Sites and Pressure Sensing Devices to Determine the Effect of Air-Fuel Equivalence Ratio on the Morphology of Knocking Combustion

In spark-ignition combustion, knocking combustion inherently presents an interaction between the main flame front and end gas autoignition. Conventionally, it generates a high amplitude pressure wave traveling across the chamber that can be responsible for reducing the performance of the engine, and can cause heavy damage to engine components. In order to study the phenomenon in a controllable way, experiments were performed on a specialized single-cylinder research engine fitted with a liner equipped with four equi-spaced spark plugs in the side so as to propagate various flame topologies from those locations, and hence achieve more controlled knock events. In addition, six pressure transducers were employed at distinct locations to precisely record details of the autoignition event by monitoring the pressure oscillations, and with them the combustion characteristics and knock intensity. Various spark ignition approaches such as the spark timing (ST), the number of ignition sites, and the location of the active spark plugs were applied to analyze the knock attributes for different ignition strategies. It has been observed that the knock intensity of single spark ignition is normally weak, while increasing the activated plug number from one to three could remarkably increase the maximum amplitude of pressure oscillation (MAPO) and knock cycle numbers. However, four spark ignition mitigates the steep pressure spikes caused by knock, and reduces the MAPO to a lower level than triple spark ignition. Since knock occurrence depends on the relative air-fuel ratio (?), this paper also described the effect of rich and lean conditions (? = 0.7, 0.9, 1.0, and 1.3), on knock instigation coupled with firing multiple spark ignition sites. The experimental results revealed that ? = 0.9 case shows the maximum knock propensity along with various high frequency acoustics modes due to higher heat release rate with faster flame propagation inside the chamber. However, for the same operating conditions ? = 0.7 and 1.0 gave slight to moderate knock.